As illustrated in FIG. 6, liquid crystal panels are fabricated by orientation-layer printing on and rubbing each of a pair of substrates 1 and 2 (a glass substrate on which array chips have been formed and an opposing glass substrate on which a color filter has been formed), followed by processing for alignment, putting together the substrates 1 and 2 with a sealant 3 there between forming a cell 4, pouring liquid crystal through an opening 5 of the sealant in between the substrates 1 and 2, and sealing in the opening 5. In FIG. 6, a temporary fixer 6 temporarily fixes the substrates 1 and 2.
Conventional methods of curing the sealant while maintaining the gap between the substrates 1 and 2 within the desired range includes a exist vacuum pack method that employs a vacuum pack, a heated-plate pressurizing method that employs a heated plate for pressurizing, and a pressurizing-while-evacuating method.
As illustrated in FIG. 7, the vacuum pack method is performed by inserting a cell 4 obtained by aligning substrates 1 and 2 and putting them together with a sealant interposed into a bag for vacuum packing, and evacuating by putting the whole bag 7 in a vacuum chamber. After a desired degree of vacuum has been reached, the bag 7 is hermetically sealed by thermally fusing an opening 7a (the dashed and dotted line of FIG. 7) restoring, atmospheric pressure to make it a pressurized state, and placing the entire bag 7 into a furnace, where the entire cell 4 is gradually heated to cure the sealant 3.
In the heated-plate pressurizing method, as illustrated in FIG. 8 and FIG. 9, a cell 4 obtained by aligning and joining substrates 1 and 2 with a sealant 3 is subsequently interposed between multi-layered heated plates 11 that vertically slide up and down and are pressed by a pressing apparatus (not shown in the drawing) with cell 4 held between the heated plates 11, to raise the temperature of cell 4 by means of a heating plate (not shown in the drawing) attached to heated-plates 11 via heated-plates 11 and curing sealant 3.
In the vacuum pressurization method, as illustrated in FIG. 10, a pair of upper and lower housing frames 17 and 18 obtained by fusing metal films 16 in the middle of a pair of approximately square outer frames 15 have an open central portion that is closely pressed from the top and the bottom with an O-ring interposed, the temperature of cell 4 is raised as it is inserted between the metal films 16, and the space between the metal films 16 of housing frames 17 and 18 is evacuated, thereby curing sealant 3. By raising the temperature, in order to avoid non-uniform heating of cell 4 because of formation of wrinkles on metal films 16, the temperature of outer frame 15 is chosen to be higher than the temperature of the metal films 16 and a tension is applied to the metal films 16. In FIG. 10, is a heater 20 installed inside outer frame 15 and a heater 21 is provided on the outside surface of metal films 16.
However, each of the conventional methods of manufacturing liquid crystal panels suffer the problems as described below.
In the vacuum pack method, as the cross sectional area of opening 5 of sealant of cell 4 is extremely small, e.g., 5 .mu.m.times.5 mm, the external pressure of cell 4 inside bag 7 rapidly decreases toward vacuum during evacuation, and at the same time the decrease in air pressure between substrates 1 and 2 is slower that of the outside because the air between substrates 1 and 2 is not rapidly exhausted, thus resulting in a higher pressure of inside cell 4 than outside of cell 4, and the swelling deformation of substrates 1 and 2 as shown in FIG. 11. Also, a volatile component generated inside cell 4 tends to remain inside cell 4, which also causes swelling deformation of substrates 1 and 2.
Consequently, the volume between substrates 1 and 2 is increased, causing further delay in evacuation. Also, deformation of substrates 1 and 2 as described above has tended to cause either deformation or peeling of sealant 3 and or temporary fixer 6, change the preset gap or the medistance of miss-alignment between substrates 1 and 2. Also, as cell 4 is not constrained when restoring the atmospheric pressure by opening bag 7, similar problems tend to be caused by the deformation of substrates 1 and 2 resulting from the difference in the modulus of elasticity due to the difference in construction of substrates 1 and 2 or from the difference in the pressure exerted on substrates 1 and 2.
Also, as the curing temperature of sealant 3 approaches to the heat-resistance temperature of bag 7, it is not always possible to raise the temperature to a value high enough to cure sealant 3, thus resulting in an insufficient curing of sealant 3 or deteriorating the alignment accuracy due to softening of temporary fixer 6.
Furthermore, there is an additional problem of not being able to maintain the gap between substrates 1 and 2 at a required distance due to an increase in the internal pressure of cell 4 which is caused by the volatile component generated inside cell 4.
In the heated-plate pressurizing method, because non-uniform thermal expansion is caused on each of substrates 1 and 2, resulting in deterioration in the accuracy of alignment, it is difficult to satisfactorily maintain the accuracy of temperature distribution on heated plates 11. Also, as the method is so configured as to press a pressing apparatus on each of substrates 1 and 2 sandwiched between heating plates 11, it suffers from problems of low processing capabilities when the entire apparatus is large scale.
In the pressurizing-while-evacuating method, although cell 4 is heated with a heater 20 via metal films 16 it cause an effect similar to directly heating cell 4 with heater 20, because the metal films 16 are thin and have a high thermal conductivity. As a result, non-uniformity of the heating temperature of heater 20 causes non-uniformity of thermal expansion on each of substrates 1 and 2, a problem deteriorating the accuracy of alignment This is similar to the heated-plate method. Also, as metal films 16 are extended under tension to the limit of their elasticity, the weld between metal films 16 and outer frame 15 fatigues, sometimes failing to pressurize as metal films 16 are broken at that section. Also, when metal films 16 are sometimes damaged and broken at the corner of cell 4 when they are pressurized by the atmospheric pressure from the outside surface toward cell 4. In order to prevent such a situation, it is often necessary to include metal frame adjusted to the external dimensions and thickness of cell 4 and place it over cell 4, which hampers efficiency. It is also necessary to incorporate thermocouples to control heaters 20 and 21, and to make wiring and piping connections for heaters 20, 21 and the thermocouples together for evacuation. When assembling a unit with a plurality of housing frames 17 and 18, the work of making these wiring and piping connections increasing indifficulty and the cost of the equipment is increased .